1. Field of The Invention
The present invention relates generally to panels used in connection with aircraft and, more particularly, to interior panels for reducing low-frequency noise.
2. Description of Related Art
A typical aircraft has a continuous diaphragm construction, which comprises a number of individual skin panels. The skin panels are secured to a frame of the aircraft and provide, among other things, an aerodynamic surface for the aircraft. As the aircraft travels through the atmosphere, a turbulence of air is generated around the skin panels. This air turbulence can set the skin panels into vibration. The skin panels often vibrate in several modes with a significant sheer wave (analogous to the shaking of a rope). A vibrating skin panel can often act as an efficient loud speaker, radiating sound to the interior panels and into the cabin of the aircraft.
Energy from the vibrating skin panels can also be transferred into the aircraft frame and, subsequently, to the interior panels of the aircraft, setting them into vibration. The vibrating interior panels can also act as loudspeakers, generating sound which is transferred into the cabin. Energy from the vibrating skin panels can also be transferred to the interior panels through the space between the skin panels and the interior panels. Thus, although the skin panels can be somewhat effective in blocking sounds generated outside of the aircraft from the interior cabin, these skin panels can transmit air turbulence sounds to the interior panels.
A typical interior panel of an aircraft may include a trim or sidewall panel, a ceiling panel, a luggage rack side wall, a lavatory side wall, a galley side wall, or a floor panel. Prior art interior panels may comprise an aluminum base with a plastic coating or, alternatively, may comprise a honeycomb composite. The honeycomb composite may comprise two fiberglass layers separated by a light layer of paper material, for example. These interior panels are typically secured to the frame of the aircraft.
Another source of sound, which can be transmitted to the interior panels, is generated primarily by engine vibrations. These engine vibrations are transmitted to the frame of the aircraft and, subsequently, transmitted to the interior panels.
A typical prior art solution for attenuating the radiation of sound, from air turbulence and engine vibrations, into the cabin of the aircraft utilizes insulation blankets, placed into an air space between the skin panels and the interior panels. These insulation blankets may comprise fiberglass wool, for example. Sound from the skin panels is transferred in a direction toward the cabin through the "pumping" of air back and forth through the absorbent layers of the insulation blankets. This pumping of air through the absorbent materials of the insulation blankets results in an attenuation of the sound energy.
Since the insulation blankets are also set into oscillation by the wave front, and since the amount of energy required to sustain this oscillating motion is proportional to the square of frequency (kinetic energy is proportional to the square of velocity), insulation blankets can be somewhat effective in insulating higher frequency sounds. Lower frequency sounds, however, are not effectively attenuated by these prior art insulation blankets. For example, the low frequency engine tones in the rear portion of a typical aircraft cannot be effectively reduced by these insulation blankets.
In addition to not significantly attenuating low frequency sounds, prior art insulation blankets have other shortcomings. For example, fuselage insulation blankets often become wet during normal flight operation, as a result of aircraft operating conditions. In addition to increasing the overall weight of the aircraft, these wet insulation blankets have introduced corrosion into the aircraft. Additionally, in some instances, airlines have experienced passenger and service door malfunctions, due to frozen insulation blankets in the door. Further, the installation of these insulation blankets between the skin panels and the interior panels can be time consuming and expensive, not to mention the costs associated with maintenance and replacement of these insulation blankets. A need has existed in the prior art for a simple structure of providing adequate sound attenuation. This structure should significantly attenuate both high and low frequency sounds, and should be capable of providing thermal insulation capabilities as well.